Combustion of high-sulfur coal in a fluidized bed reactor

ABSTRACT

Heating values are recovered from sulfur-containing coal in a reactor which includes a gasification zone, a combustion zone and a heat recovery zone. A fluidized bed of coal and limestone is maintained within the gasification zone, which is supported by a moving foraminous grate. Primary air, which is also the fluidizing medium, enters the gasification zone through the grate. The zone is operated adiabatically, under reducing conditions, to yield CaS and an effluent rich in CO. The CaS is discharged from the gasification zone by the moving grate and the gaseous effluent, containing entrained desulfurized coal fines, enters the combustion zone where secondary air is added. The exotherm of combustion is retrieved in the heat recovery zone, using conventional techniques. The flue gas from the reactor contains little sulfur dioxide to pollute the environment. The CaS may be processed in several ways to recover elemental sulfur.

United States Patent 72] inventor Marshall L. Spector Bellemead, NJ.[21] App]. No. 135,981 [22] Filed Apr. 21, 1971 [45] Patented Dec. 7,1971 [73] Assignee Air Products and Chemicals, Inc. Allentown, Pa.Continuation-impart of application Ser. No. 60,379, Aug. 3, 1970, nowPatent No. 3,599,610, dated Aug. 17, 1971. This application Apr. 21,1971, Ser. No. 135,981

[54] COMBUSTION OF HIGH-SULFUR COAL IN A FLUlDlZED BED REACTOR 10Claims, 2 Drawing Figs.

[52] [1.8.01 110/1 J, 110/1 K, 110/28 .1, 122/4 [51] Int. Cl F23d 19/00[50] Field of Search 122/4 D; 110/1 J, 1 K, 28.]

[5 6] References Cited UNITED STATES PATENTS 3,080,855 3/1963 Lewis110/1 X 3,320,906 5/1967 Domahidy 1 10/1 3,540,387 ll/197O McLaren etal. 110/1 FOREIGN PATENTS 619,117 4/1961 Canada llO/l Primary Examiner-Kenneth W. Sprague Anorneys-Barry Moyerman and B. Max Klevit ABSTRACT:Heating values are recovered from sulfur-containing coal in a reactorwhich includes agasification zone, a combustion zone and a heat recoveryzone. A fluidized bed of coal and limestone is maintained within thegasification zone, which is supported by a moving foraminous grate.Primary air, which is also the fluidizing medium, enters thegasification zone through the grate. The zone is operated adiabatically,under reducing conditions, to yield CaS and an effluent rich in CO. TheC215 is discharged from the gasification zone by the moving grate andthe gaseous effluent, containing entrained desulfurized coal fines,enters the combustion zone where secondary air is added. The exotherm ofcombustion is retrieved in the heat recovery zone, using conventionaltechniques. The flue gas from the reactor contains little sulfur dioxideto pollute the environment. The Gas may be processed in several ways torecover elemental sulfur.

PATENTED mic Han 3.625; 164

' sum 2 OF 2 Fig. 2

APPROXIMATE MINIMUM MOLE RATIO OF CO/CO NECESSARY IN GASIFICATION ZONEOFF'GAS TO PREVENT OXIDATION THERE IN OF 008 TO C030 Co s0 MOLE RATIO OF00/00:

I600 I800 2000 am 2200 GASIFIER TEMP. F

INVENTOR. Marshall L.Spector BY 0 0w ATTORNEY COMBUSTION OF HIGH-SULFURCOAL IN A FLUIDIZED BED REACTOR CROSS-REFERENCE TO RELATED APPLICATIONSThis application is a continuation-in-part of application Ser. No.60,379, filed Aug. 3, 1970, now U.S. Pat. No. 3,599,610 issued Aug. 17,1971.

BACKGROUND OF THE INVENTION 1. Field of the Invention Generally, thisinvention relates to the desulfurization of coal. More particularly, itpertains to a process for the combustion of sulfur-containing coalwherein the sulfur is removed as calcium sulfide and the flue gas,consequently, contains minimal S0,.

2. Prior Art The prior art is legion and it would indeed be presumptuousto attempt a summary herein which would be both comprehensive andobjective.

Combustion of coal in fluidized beds is not new. It is shown, forexample, in U.S. Pat. No. 3,437,561. Recent work along these lines, doneby the British Coal Utilization Research Association, is described in anarticle by S. Wright et al. II. of the Institute of Fuel 42, 235-239,June, 1969. There have also been prior attempts to recapture the sulfurcontent of coal, as calcium sulfate, in the course of single-stagecombustion of coal in fluidized bed reactors. These were described,variously, by E. B. Robison et al. in Fluidized Combustion of Coal forBoiler Capital Cost Reduction and Air Pollution Control" 'Preprint 45C,A.I.Ch.E., New York (March, I969) and by C. W. Zielke et al. in SulfurRemoval during Combustion of Solid Fuels in a Fluidized Bed of Dolomite"Reprints A.C.S., Division of Fuel Chemistry, Vol. 13, No. 4, New York(Sept., 1969). One of the most interesting of the prior art processes isthe Winkler process in which fine fuel is gasified in a fixed fluidizedbed. This process is discussed in Industrial and Engineering Chemistry,Vol. 40, No. 4 (Apr., I948) page 562 et seq.

The recent emphasis on preservation of our environment has now made itextremely desirable to obtain heat values from carbonaceous fuelswithout polluting the atmosphere with sulfur dioxide. While one obvioussolution is to utilize fuels which contain little or no sulfur, suchfuels are scarce and relatively expensive. Prior art technology, eventhat devoted to removal of sulfur from carbonaceous fuels, was moreconcerned with upgrading the quality of the carbonaceous material thanwith minimizing the ecological trauma associated with its combustion.The treatment or pretreatment of such materials with various sulfuracceptors or getters" is discussed inter alia in U.S. Pat. Nos.2,824,047 and 3,387,941. However, in the former, regeneration of theacceptor yields S and, in the latter, sulfur-derived impurities arereleased in gaseous form. The general technology of attempts todesulfurize fuels undergoing gasification is reviewed in an article byA. M. Squires in Preprints"Division of Fuel Chemistry, A.C.S., Vol. 10,No. 4, pages 20-41 (1966), which discloses, inter alia, the use ofdolemite for desulfurization. Said article is incorporated herein byreference.

In the parent applications referenced above, sulfur-containing coal istreated in a two-stage process which minimizes evolution of volatilesulfur compounds. In the first stage, sulfur-containing coal is reactedwith CaO in a fluid bed gasifier, which operates adiabatically underreducing conditions, to yield CaS and a gas rich in CO. The second stageis a conventional boiler in which the off-gas from the first stage andthe coal fines entrained therein are delivered to the boiler as fuel.Flue gas from this boiler contains substantially no sulfur dioxide as apollutant. CaS is withdrawn from the first stage as a valuablebyproduct.

The prior art relative to coal gasification discloses the concept ofsupporting a fluidized bed on a continuous foraminous moving gratethrough which the fluidizing gas enters the bed. Preferably, the grateis divided into sections so that air flow can be balanced to compensatefor varying bed depth. Such a structure is shown, for example, GodelU.S. Pat. Nos. 2,866,696 and 3,302,597.

The instant application is a logical outgrowth of these twotechnologies. In it, the two stages of the parent application arecombined in a single reactor which contains within it a gasificationzone, a combustion zone and a heat recovery zone. Thus there is no needfor separate gasifier and boiler structures. Further, use of the priorart moving grate permits ready removal of ash and CaS from thegasification zone, within which reducing conditions are maintained.

SUMMARY OF THE INVENTION Combustion of sulfur-containing coal iseffected in a plurality of zones contained with a single reactor. Thefirst zone is a fluidized bed gasification zone, wherein the bed ispreferably supported on a travelling grate. Within this zone, which isoperated under reducing conditions, CaO (or limestonelike materialscapable of yielding CaO), which is added along with the carbonaceousfuel, reacts with the sulfur in the fuel to yield CaS. Operatingconditions are set which maintain the reducing environment and preventoxidation of the desirable CaS to the undesirable CaSO The zone containsa steady state mixture of presized limestone (in stoichiometric excessof that needed to react with the sulfur in the fuel) and solidcarbonaceous fuel. Reaction temperatures range from about l,650 F. toabout 2,200 F. The products of the first zone are both solid andgaseous. Ash, CaS and some fuel are withdrawn by the moving grate which,in effect, scrapes the bottom of the bed to remove larger and/or heaviersolids. Elemental sulfur can be readily recovered from the CaS. Anoff-gas, rich in CO and H rises to the next zone in the reactor carryingwith it, elutriated fines of reduced sulfur content, fly ash and smallamounts of CaO and CaS.

Subsequent zones constitute, in effect, a boiler housed in the samereactor shell. In these stages, the heat content of the gases and solidsfrom the first zone is released and recovered. In a combustion zone,which adjoins the gasification zone, secondary air is admitted for thepurpose of fully completing the oxidation of both solid and gaseousgasifier effluent. There is subsequent recovery of heat in aconventional heat recovery zone. The flue gas from the recovery zoneconsists primarily of water, carbon dioxide and nitrogen. It containsnegligible SO, compared to that which would have been present had theoriginal sulfur-containing fuel been burned in a conventional manner.

Accordingly, it is an object of the invention to provide a methodwhereby heat values can be recovered from sulfurcontaining fuel withminimal production of environment-contaminating S0 Another object of theinvention is to provide a method of burning high-sulfur coal whichutilizes a series of transversely extending longitudinally aligned zoneswhich together perform the functions of a gasifier and a boiler.

Other objects of the invention will be apparent to those skilled in theart from a consideration of the description which follows, when read inconjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING In the drawing, wherein like referencenumerals indicate like parts:

FIG. 1 is a schematic flow diagram of a preferred embodiment of theinvention; and

FIG. 2 is a graph illustrating the minimum CO/CO mole ratio which mustbe maintained in the first zone off-gas, as a function of gasifiertemperatures, to produce CaS rather than CaSO DESCRIPTION OF THEPREFERRED EMBODIMENT Referring now to FIG. 1, there is shown a reactor10 in which heating values are recovered from coal containing sulfur, ina manner which optimally minimizes efflux of volatile sulfur compoundsto the external environment. In general, the process of heat recoveryfrom the coal is accomplished within the single apparatus 10, havingthree sequential contiguous and generally horizontal overlying zonesthrough which the combustion and heat recovery process takes placebefore flue gas is released through the stack or outlet line 18. Thesulfurcontaining coal is passed into a gasification zone 12 whereinitial combustion is carried out in a fluidized bed 39 which issupported by a movable foraminous grate 20, as will be described indetail in the following paragraphs. The gaseous products of initialcombustion then pass upwardly into a second stage combustion oroxidation zone 14 where secondary air is admitted into the reactorthrough secondary air inlet 22, as shown. The two-stage combusted coalproducts then pass through a heat recovery zone 16 and finally exit fromreactor 10 through the outlet line 18.

The reactor 10 generally extends in a substantially vertical direction24 and includes a coal inlet opening 30 passing through lateral wall 32to gravity direct or feed a mixture of calcium oxide andsulfur-containing coal from an externally located storage reservoir 28into gasification zone 12. The mixture of coal and calcium oxide,preferably in the form of limestone, is maintained within the storagereservoir 28 until needed for reaction purposes within the gasificationzone 12 of reactor 10. The coal stored within the reservoir 28 is groundto predetermined size so as to be easily fluidizable when fed into thegasification zone 12. The amount of limestone in the mixture is inexcess of that needed to produce calcium sulfide from the sulfurcontained in the coal.

The gasification zone 12 is geometrically defined in a lower surfacethereof by the moveable foraminous grate as shown in FIG. 1. Zone 12extends vertically above the fluidized layer free surface 38 andterminates in a generally horizontal plane surface contiguous to thesecond stage combustion zone 14.

The mechanical grate 20 extends on a continuous closed contour trackaround first and second driving rollers 34 and 36 secured to supportmembers 35 and 37 respectively, which are secured to a suitable wallsurface of reactor 10. Rotation of the driving rollers 34 and 36 in aclockwise direction, therefore, provides a substantially inclinedtransverse or horizontal movement 26 to the bottom of the fluidized bed39 and transports agglomerates to the chute or conduit 53. As shown inFIG. 1, first driving roller 34 is positioned below second opposingdriving roller 36 to define the grate 20 in an inclined plane withrespect to the horizontal direction 26. One of the rollers 34 or 36 maybe driven by a variable speed motor (not shown), whereas the remainingroller may be freely rotatable. By manipulation of the flow rate of coaland calcium oxide from storage reservoir 28 as well as the linear speedof the grate 20, the fluidized layer boundary surface plane may bemaintained at substantially constant height.

Between the upper and lower tracks 42, 44 of foraminous grate 20 thereare included a plurality of air boxes 40a 40p. Primary air is introducedunder pressure into plenum 48 through primary air inlet 46, located at apoint substantially below foraminous grate 20. The high-pressure airpasses from plenum 48 in a vertical direction 24 through the pluralityof air boxes 40a 40p within grate 20. These elements permit pressureregulation of the vertically directed primary air which maintains thecontents of gasification zone 12 in fluidized suspension. The flow ofair is largest through box 40a and least through box 40p, to compensatefor the decreasing depth of the fluidized bed 39 above upper track 42.in this manner provision is made for approximating a unit distributionof gas flow throughout the horizontal direction 26 of the grate 20passage. This incorporation of air in the manner described enables thefluidized bed 39 to maintain an approximate uniform degree offluidization in its transverse dimension with the boundary surface 38remaining in a substantially horizontal plane.

The movable grate 20 is generally of the standard type, well describedin the above-mentioned prior art patents. The upper and lower tracks 42,46 are constructed of cast iron, or some like material, and are formedinto a chainlike extension having air passage openings to permit thevertical flow of primary air. Projections or other vertically orientedextensions may be formed on the track surfaces in contact with thebottom of fluidized bed 39 when the inclination plane of the foraminousgrate 20 is sufficient to overcome the frictional contact force betweenthe contiguous surfaces. The projections prevent sliding of the gatheredagglomerates in a direction toward the first driving roller 34 and arefound to be of use when the inclined angle is substantially in excess of30, although the exact angle is a function of the physical properties ofthe contiguous materials and, as such, may vary over a considerablerange.

As has been briefly described, coal and calcium oxide are transported tothe gasification zone 12 from an externally located storage reservoir28. The incoming mixture may be gravity distributed on the fluidizedlayer free surface 38 by star feeder or shovel wheel 50 positioned atinlet opening 30 on the lateral wall 32 as shown in FIG. 1.Alternatively, the feed may be added directly to the body of the bed.The calcium oxide and coal mixture contained in reservoir 28 is premixedin accordance with the predetermined sulfur content of the coal, e.g.,approximately a 50 percent excess of stoichiometric amount necessary toeffect the reaction resulting in the formation of calcium sulfide andwater from calcium oxide and hydrogen sulfide. In general, the limestoneused in combination with the coal must always be in excess of thatnecessary to supply CaO in sufficient quantity to react with the sulfurin the coal. Molar excesses of 50 to 200 percent have been found to bepreferable with an optimized range between 75 and percent.

At or near the second driving roller 36, the upper track 42 verticallyemerges from the fluidized layer free surface 38 thus carrying theagglomerates from the bottom of fluidized bed 39 into the solid drawoffzone 52. The solids comprising a mixture of ash, CaO and CaS are drawnofl through solids conduit 53 to a sulfur recovering zone (not shown).It is essential that the drawoff occur within the reducing atmosphere ofzone 12.

Solid and gaseous products are combusted in the initial combustion orgasification zone 12 and pass vertically into the second stagecombustion zone 14. In order that zone 12 produce CaS rather than CaSOthe off gas moving intrazone must contain CO/CO in mole ratio shown inFIG. 2.

As has been described, secondary air inlets 22 incorporate air into zone14 to aid in an optimized combustion therein. Heat of combustion isrecovered within heat recovery zone 16, through utilization ofconventional heat exchange equipment and the flue gas leaves the reactor10 through stack or outlet line 18. In some instances where there isstill a sizeable quantity of solid combustibles in the exiting gas, suchcombustibles may be recycled to the first stage gasification zone 12within reactor 10.

Discharge to the external environment or atmosphere through the stack18, where solids recycle is desired, may be preceded by conventionalheat recovery and mechanical and/or electrostatic solids removalsystems. In this operation, it has been found that electrostaticprecipitators are particularly effective due to the high conductivityfound in carboncontaining solids.

Typical operation of the embodiment as shown in FIG. 1 is based on lton, i.e., 2,000 lbs. of coal having a sulfur content of 4.5 weightpercent with the gasifier zone 12 operating at a temperatureapproximating 2,l50 F. The influx of the coal and limestone mixture fromthe storage reservoir 28 to the gasification zone 12 totals about 2,420lbs. The solid material enters at an ambient temperature ofapproximately 77 F. and its analysis in lbs. and lb./moles is asfollows:

Primary air enters the gasifier 12 through air inlet 46 to provide 64lb. moles of oxygen and 221 lb. moles of nitrogen input into the plenum48. The fluidized bed 39 contains a steady-state mixture of ash,limestone, CaS and coal wherein the size of the limestone is adjusted toavoid excessive elutriation from the bed 39. In operation, limestonecarryover is maintained at minimal quantities, e.g., below 2 percent ofthe inventory. The limestone carryover is minimal if the particulatelime or lime-yielding material is charged having its size range from 6-9mm. in diameter combined with a superficial gas velocity through the bed39 of less than 25 ft./sec.; preferably about 14 ft./sec.

The coal within the storage reservoir 28 is preground so that 99 percenthas a diameter size less than 9-10 mm. As gasification proceeds, a givenparticle is consumed until it is finally entrained and leaves zone 12and enters the second stage combustion zone 14.

The maximum size of coal and other solid particles elutriated is afunction of the linear velocity in the bed 39. Optimized conditions areachieved when the coal has a residence time in the bed 39 just longenough to substantially reduce its particle size to the point where itis elutriated into the second stage combustion zone 14. A carryover ofcoal from zone 12 to zone 14 between -20 is usually found. Coal solidswhich are not reacted in the second stage combustion zone 14 may beultimately collected in electrostatic precipitators, as previouslydescribed, and returned to the first gasification zone 12 throughconventional means. However, in this milieu, carryover is limited by thedesiderata that (a) the temperature of the gasification zone 12 bemaintained within the range where the desired ash agglomeration occurs;(b) sulfur content of the coal elutriated into combustion zone 14 iswithin the generally recognized range for low-sulfur carbon fuels, e.g.,lessthan 1 percent by weight of the carbon content of the coal.

Solids drawoff begins when the upper track 42 of the moveable grateemerges beyond the level of the fluidized layer free surface 38 andenters drawofi zone 52 where the ratio CO/CO, is as shown in FIG. 2. Byproper adjustment of primary airflow through air boxes 40a 40p(including selective orifices) within grate 20, the material in bed 39above air box 40p achieves substantially complete gasification. However,of critical consequence is that not so much primary air be passedthrough the air boxes 40a 40p such as to allow the CO/CO ratio to dropbelow the line shown in FIG. 2.

FIG. 2, to which reference has previously been made, is a graph showingthe approximate minimum mole ratio of CO/CO,, as a function of thegasifier zone temperature, necessary to prevent oxidation of CaS toCaSO,. Graph contour line 54 represents the boundary line region betweenthe upper region 56, wherein CaS is produced, and the lower region 58,where CaSO, is produced. As shown, the critical mole ration of CO/COranges between approximately 0.87 and 1.2 within a related range ofgasifier temperatures extending from l,600 to 2,200 F. In addition, thecritical mole ratio is seen to monotonically increase in value as afunction of higher gasifier temperatures, with contour line 54approximating a linearly rising function after the gasifier has reacheda value of about 1,800 F.

At no time during operation may the partial pressures of carbon monoxideand carbon dioxide, within zones 12 and 52, be such that the criticalmole ratio of CO/CO, (as defined by boundary line 54 of FIG. 2) is lessthan the indicated value. The value of this ratio is essential andcritical to the success of the process of this invention, in order topreserve the sulfur values as sulfide and not permit their conversion toa sulfate. This process permits a relatively easy recovery of elementalsulfur from the sulfide, whereas the recovery of elemental sulfur fromsulfates may only be accomplished through an expensive reduction processwherein calcium sulfate is reduced to calcium sulfide. In the process asherein defined, calcium sulfide is the direct product of the process.

In addition to some unreacted CaO, about 2.3 lb. moles of CaS and 1.6lb. moles of ash, expressed as calcium silicate, are drawn off throughsolids conduit 53. Though the material drawn off is primarily calciumsulfide and ash, some unreacted coal may be withdrawn, which may beseparated and recycled. The calcium sulfide produced in the process maybe utilized as such or as the feed for a process to recover elementalsulfur, using any one of several methods.

One method of recovery is to drop the hot solids into water contained ina closed vessel at atmospheric pressure and sparge into the water anappropriate amount of C0,, preferably derived from plant flue gas. Thereaction converts calcium values to calcium carbonate and evolves H Squantitatively. The H S is readily recovered and can be converted toelemental sulfur using the Claus process. In this method, the ash andcalcium carbonate are removed together.

Another method is to quench the hot solids in a body of water maintainedat atmospheric conditions. Under these conditions, 11,5 and water vaporare released and calcium hydroxide and inerts are recovered as solids.

Still another method, which offers potential for recovery of calcium,while still permitting recovery of sulfur values is to collect the hotsolids in water at atmospheric pressure and, subsequently, heat thesystem to about l2S-200 C. in a pressure vessel. Under these conditions,the sulfide is largely hydrolyzed to soluble calcium hydrosulfide whichcan be separated from ash and other solids by filtration. If desired,calcium hydrosulfide can be produced at a lower temperature by theaddition of H 8. Following filtration, water and H 8 are stripped fromthe liquid phase and calcium hydroxide slurry recovered. The slurry issufficiently free of ash so that it can be recycled for ultimateaddition to hopper 28.

Alternatively, the filtrate may be treated with CO; at lowertemperature, to release H 8 and a calcium carbonate slurry recovered.The slurry is sufficiently free of ash so that it can be recycled forultimate addition to hopper 28, just as the slurry in the aboveparagraph.

The gaseous components which enter the second stage combustion zone 14comprise 136 lb. moles of CO, 35 lb. moles of H 0.2 lb. moles of 11:0,0.4 lb. moles of C0 221 lb. moles of N and traces of H 8. Due to thenature of the combustion system, these components cannot be measureddirectly but are calculated on the basis of the restricted airflow fedto the first stage combustion zone 12. The second stage combustion zone14, includes incorporation of secondary air entering through the inlets22, which is preferably preheated to achieve maximum second stagecombustion.

Flue gas being emitted from the stack 18 contains approximately 136 lb.moles of CO 35 lb. moles of H 0, 563 lb. moles of N and 0.5 lb. moles ofS0,. The process as herein detailed, therefore performs as if a fuelhaving 0.8 percent sulfur were being burned conventionally whereas, inactuality, the fuel contained 4.5 percent sulfur. The essential successof the process is attributable to the operation of the gasifier 12 underthose critical reducing conditions indicated in FIG. 2. Accordingly, thefeed of air and coal must be adjusted so that the CO/CO ratio isrepresented by a point on or above the boundary line 54 shown in FIG. 2.

While a preferred embodiment of the invention has been described, manymodifications and variations will be apparent to those knowledgeablewith respect to the design and operation of fluidized bed gasifiers.Accordingly, the scope of the invention is to be limited only by areasonable interpretation of the appended claims.

What is claimed is:

l. A process for burning sulfur-containing coal with minimal pollutionof the environment by S0 comprising:

a. establishing a plurality of generally horizontal overlying zoneswithin a vertically extending reactor, said zones including a lowergasification zone, an upper heat recovery zone and an intermediatecombustion zone;

b. maintaining a bed of particulate coal and limestone within saidgasification zone;

0. fluidizing said bed with a stream of primary air;

d. feeding said bed with fluidizable size sulfur-containing coal andwith an amount of particulate limestone in excess of that needed toproduce CaS from the sulfur contained in the coal;

. reacting carbon with oxygen in said bed to yield CO;

adjusting the feed of primary air and coal to produce and maintain areducing environment within said gasification zone, as reflected by theCO/CO, ratio in the gaseous effluent therefrom, said ratio having avalue above the minimum represented by the curve in FIG. 2, wherebyoxidation of CaS to CaSO is prevented; and

g. withdrawing CaS from the gasification zone as a solid and evolvingfrom said zone, as a gaseous effluent, a mixture comprising CO and H inwhich are elutriated coal fines having a sulfur content less than thatof the coal originally fed to the gasification zone.

2. The process of claim 1 wherein said fluidized bed is maintained on amoving foraminous grate and the fluidizing air enters the bed throughthe grate.

3. The process of claim 2 wherein said grate moves transversely andupwardly, discharging ash and CaS at its higher elevation.

4. The process of claim 3 wherein different quantities of primary airare fed through different portions of said grate so that, despite thevarying depth of bed across the transverse extent of the grate, thelinear velocity of fluidizing air flowing therethrough is maintainedsubstantially uniform.

5. The process of claim 1 which includes the additional steps of:

h. adding secondary air to said combustion zone to oxidize the effluentfrom said gasification zone with a consequent release of heat;

i. recovering substantially all of the combustion zone exotherm in saidheat recovery zone; and

j. venting from the heat recovery zone a flue gas containing minimalquantities of S0,.

6. The process of claim 5 wherein said fluidized bed is maintained on amoving foraminous grate and the fluidizing air enters the bed throughthe grate.

7. The process of claim 6 wherein said grate moves transversely andupwardly, discharging ash and CaS at its higher elevation.

8. The process of claim 7 wherein different quantities of primary airare fed through different portions of said grate so that, despite thevarying depth of bed across the transverse extent of the grate, thelinear velocity of fluidizing air flowing therethrough is maintainedsubstantially uniform.

9. The process of claim 8 wherein the amount of limestone fed in step(d) ranges from 50 to 200 mol percent in excess of the stoichiometricquantity.

10. The process of claim 9 wherein about 99 percent of the coal fed tothe gasification zone has a size less than l0 mm.

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,5, m December 7, 1971 Inventor(s) Marshall L. Specter It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 2, line 16 "with" should read "--within--" Column 5, line 27after "zone 1 insert "--of--" Column 5, line 27 after "5-20 insertSigned and sealed this 30th day of May 1972.

(SEAL) Attest:

EDWARD M.FLETCHER, JR. Attesting Officer ROBERT GOTTSCHALK Commissionerof Patents

2. The process of claim 1 wherein said fluidized bed is maintained on amoving foraminous grate and the fluidizing air enters the bed throughthe grate.
 3. The process of claim 2 wherein said grate movestransversely and upwardly, discharging ash and CaS at its higherelevation.
 4. The process of claim 3 wherein different quantities ofprimary air are fed through different portions of said grate so that,despite the varying depth of bed across the transverse extent of thegrate, the linear velocity of fluidizing air flowing therethrough ismaintained substantially uniform.
 5. The process of claim 1 whichincludes the additional steps of: h. adding secondary air to saidcombustion zone to oxidize the effluent from said gasification zone witha consequent release of heat; i. recovering substantially all of thecombustion zone exotherm in said heat recovery zone; and j. venting fromthe heat recovery zone a flue gas containing minimal quantities of SO2.6. The process of claim 5 wherein said fluidized bed is maintained on amoving foraminous grate and the fluidizing air enters the bed throughthe grate.
 7. The process of claim 6 wherein said grate movestransversely and upwardly, discharging ash and CaS at its higherelevation.
 8. The process of claim 7 wherein different quantities ofprimary air are fed through different portions of said grate so that,despite the varying depth of bed across the transverse extent of thegrate, the linear velocity of fluidizing air flowing therethrough ismaintained substantially uniform.
 9. The process of claim 8 wherein theamount of limestone fed in step (d) ranges from 50 to 200 mol percent inexcess of the stoichiometric quantity.
 10. The process of claim 9wherein about 99 percent of the coal fed to the gasification zone has asize less than 10 mm.